Process for the Aerobic-Thermophilic Stabilization and Disinfection of Sludge

ABSTRACT

Disclosed is a method for the aerobic-thermophilic stabilization and decontamination of sludge, in which a) raw sludge having a dry substance concentration of 3 to 7 percent by weight is fed to a first stage and remains there for an average dwell time of four to ten days at minimum temperatures of 42° C. while an oxygen-containing gas is added so as to obtain partially stabilized sludge, b) said partially stabilized sludge is fed to a second stage in which the same is further stabilized and decontaminated for an average dwell time that amounts to 30 to 70 percent of the average dwell time of the first stage at temperatures exceeding 500° C. while adding an oxygen-containing gas.

The invention relates to a process for the stabilization anddisinfection of sludge in two steps.

A generic process has been described, for example, in “KorrespondenzAbwasser”, 29th year, issue April 1982, pp. 203-207. In this process,the entering raw sludge, after having been gravity-thickened, is aeratedwith simultaneous intense mixing in two heat-insulated reactorsconnected in series. In the reactor I, the temperature variation ispredominantly around the upper mesophilic range (30≦t<42° C.), whereasin the reactor II thermophilic temperatures (≧42° C.) are permanentlymaintained. With sufficient retention time of each batch in reactor IIand maintenance of the temperatures above 50° C., disinfection isachieved. In this plant, a batch of stabilized and disinfected sludge isdischarged from reactor II once daily, followed by transferring acorresponding batch from reactor I to reactor II and finally fillingreactor I with raw sludge. This operation is required to ensuredisinfection in reactor II and has been regular practice to date.

Drawbacks of this process are high variations of temperature, oxygendemand and in some cases pH-value in reactor I and the temporarilyoccurring odor emissions, since 30 to 50% of the reactor contents areusually replaced by raw sludge at once when the tank is charged. Forlarger plants with three reactors, these problems can again aggravate.

DE PS 39 05 228 discloses a process for the aerobic stabilization anddisinfection of thickened sewage sludge in several steps, using one ormore heat-insulated tanks, preferably with stirring, with the additionof oxygen-containing gas. In this process, the entering thickened rawsludge is to be supplied to a stage A which can be aerated andoptionally agitated, and heated to temperatures of at least 25° C. Fromthis stage, it is to be transferred batch by batch into a stage B whichcan be aerated and/or agitated, in which temperatures of above 50° C.are generated essentially by aerobic degradation and wherein the timebetween two batches is sufficient for disinfection depending ontemperature. From this stage, a batch of sludge disinfected in stageB—which corresponds to the batch respectively transferred from stage Ato stage B—is to be transferred to a stage C, which can be aeratedand/or agitated, in which the temperatures are reduced to the range from25 to 45° C., preferably about 35° C., by heat exchange. Always, thetransferring of the batch is to start with the discharge of thecorresponding batch from stage C, to be continued by transferring thecorresponding batch from stage B into stage C, to be continued bytransferring the corresponding batch from stage A into stage B, followedby charging stage A with raw sludge again. In this process, the averageretention time should preferably be about one day in stage A, preferablyfrom 2 to 3 days in stage B, and from 3 to 4 days in stage C.

In practice, it has been found disadvantageous with this process thattypically about 100% of the contents of the tank is exchanged once a dayin stage A, and from 33 to 50% of the contents of the tank in stage B.This causes heavy variations of temperature and oxygen content in theupstream stages, and temporarily odor emissions, especially in theregion of stage A, the temperature of which is always below thethermophilic range.

Therefore, it is the object of the invention to develop a process forthe stabilization and disinfection of thickened sludge which overcomesthe drawbacks of the prior art.

This object is achieved by a process for the aerobic-thermophilicstabilization and disinfection of sludge wherein

-   a) raw sludge having a dry matter content of from 3 to 7% by weight    is fed to a first stage, where said raw sludge remains during an    average retention time of from four to ten days at temperatures of    at least 42° C. with an introduction of an oxygen-containing gas to    obtain a partially stabilized sludge;-   b) said partially stabilized sludge is fed to a second stage in    which said sludge is further stabilized and disinfected during an    average retention time, which is 30 to 70% of the average retention    time of the first stage, at temperatures of above 50° C. with the    introduction of an oxygen-containing gas.

Thus, according to the invention, the sludge is always kept in thethermophilic range, i.e., at temperatures of at least 42° C., preferablyof at least 45° C., in the first stage. With average retention times offrom 4 to 10 days, this results in a partial stabilization of thesludge. The stabilization and disinfection are then completed in thesecond stage, also in the thermophilic range.

A sludge is considered stabilized if it can be stored for an extendedperiod of time or be applied to agricultural land without problems. Thiscondition is typically achieved if the organic content is reduced byabout 30 to 50%, preferably more than 40%. The organic content isdetermined by the loss on ignition of a dried sample according to DIN38409.

While in the prior art the temperatures in the first tank usually varyabout the upper mesophilic range, the process according to the inventiontakes care that the temperature does not fall below the thermophilicrange, i.e., below 42° C., after the feed of the raw sludge. Preferably,the temperatures in this stage are at least 45° C., more preferably atleast 48° C.

In the process according to the invention, discharging and chargingcycles are run which usually begin with discharging from the secondstage. Subsequently, a corresponding batch of partially stabilizedsludge is transferred from the first stage to the second stage, and thenraw sludge is fed into the first stage.

In usual plants, such a charging cycle is run once a day. Depending onthe kind of plant, it may also be possible to perform charges twice aday. Especially, when charging is performed manually, it may happen thatthere is no charging occasionally, for example, there is often nocharging on Sundays and public holidays.

According to the invention, it is preferred that charging takes place ona regular basis, i.e., preferably once daily, because it is only thenthat from 10 to 25% of the contents is replaced by raw sludge in thefirst stage. According to the invention, if possible, not more than 30%,preferably less than 25%, should be replaced by raw sludge in the firststage in order to avoid odor emissions.

In a preferred embodiment of the invention, the average retention timein the first stage is from 5 to 6 days. The preferred average retentiontime in the second stage is from 2.5 to 3 days.

The first and second stages are performed in separate tanks. Either thefirst tank may be larger than the second tank or for the first stagetanks of the same size as those used in the second stage are employed,but more of them, for example, two tanks for the first stage and onetank for the second stage. By doing so, it is achieved that the averageretention time in the second stage is about 50% of the retention time inthe first stage. In one embodiment, the retention time in the secondstage is clearly shorter than that in the first stage, for example, in arange of from 30 to 40%. In another embodiment, the average retentiontime in the second stage is within a range of from 60 to 70% of theretention time of the first stage. More preferably, the averageretention time in the second stage is within a range of from 40 to 60%of the retention time of the first stage.

In accordance with this application, a tank is any suitable container,including a basin. Such containers are also referred to as reactors.

In the first and second stages, an oxygen-containing gas, such as air,is supplied. As in the prior art plants, the aeration intensity,aeration time and/or oxygen content of the supplied gas is controlled.Typical measured and controlled quantities for the oxygen supply are thebatches of raw sludge, the redox potential or oxygen content in thesludge, and the oxygen content or CO₂ content of the exhaust gas.

The temperatures of the first and second stages can be controlled bysupplying or withdrawing heat. For example, heat exchangers may beemployed for this purpose. In a preferred embodiment, the temperature isnot above 60° C. in the first stage and not above 65° C. in the secondstage. Particularly, suitable temperatures for the second stage areabout from 55 to 60° C. Undesirable heat losses can be avoided by asufficient insulation of the tanks.

The stabilized and disinfected sewage sludge obtainable by this processmay subsequently be further treated physically, chemically orbiologically. It is highly suitable for land application.

Exhaust gas released from the process may also be recovered and treatedphysically, chemically and/or biologically.

As the sludge for the process according to the invention, thickenedsewage sludges may be used, as obtained from the mechanical orbiological treatment of domestic and/or industrial sewage. Thethickening may be effected by gravity or mechanically.

The process may also be employed for the stabilization and disinfectionof liquid manure, other organic concentrates, for example, from foodprocessing, and food wastes.

According to the invention, the solids content is selected in such a waythat, on the one hand, biologically degradable organic matter issufficiently present, and on the other hand, the rheological propertiesare still of a nature that the sludge is easy to handle. Especially, forslightly higher solids contents of the raw sludge, for example, aboutfrom 4 to 5%, it is usually not required to heat stage A because thethermophilic temperatures are reached autothermally.

1. A process for the aerobic-thermophilic stabilization and disinfectionof sludge, wherein: a) raw sludge having a dry matter content of from 3to 7% by weight is fed to a first stage, where said raw sludge remainsduring an average retention time of from four to ten days attemperatures of at least 42° C. with the introduction of anoxygen-containing gas to obtain a partially stabilized sludge; b) saidpartially stabilized sludge is fed to a second stage in which saidpartially stabilized sludge is further stabilized and disinfected duringan average retention time which is 30 to 70% of the average retentiontime of the first stage at temperatures of above 50° C. with theintroduction of an oxygen-containing gas.
 2. The process according toclaim 1, wherein the average retention time in the first stage is fromfive to six days.
 3. The process according to claim 1, wherein theaverage retention time in the second stage is from 2.5 to three days. 4.The process according to claim 1, wherein the first and second stagesare performed in separate tanks.
 5. The process according to claim 4,wherein the number of tanks for the first stage is higher than thenumber of tanks of the second stage.
 6. The process according to claim1, wherein air is supplied as said oxygen-containing gas in the stages,wherein the aeration intensity and/or aeration time of the supplied gasare controlled.
 7. The process according to claim 6, wherein the amountof raw sludge, the redox potential or oxygen content in the sludge andthe oxygen content or CO₂ content of the exhaust gas are employed asmeasured and controlled quantities for the oxygen supply.
 8. The processaccording to claim 1, wherein the temperature in the first and secondstages is controlled by supplying or withdrawing heat.
 9. The processaccording to claim 1, wherein the temperature in the first stage doesnot exceed 60° C.
 10. The process according to claim 1, wherein thetemperature in the second stage does not exceed 65° C.
 11. The processaccording to claim 1, wherein the temperature in the second stage iswithin a range of from 55 to 60° C.
 12. The process according to claim1, wherein the temperature in the first stage is at least 45° C.
 13. Theprocess according to claim 1, wherein the stabilized and disinfectedsludge is subsequently further treated physically, chemically and/orbiologically.
 14. The process according to claim 1, wherein exhaust gasreleased from the process is recovered and treated physically,chemically and/or biologically.